Nice to see another Exchange bug (not confirmed with Microsoft) in practice. An user could not send message and accept invitations only with two internal recipients (all users from Exchange 2010 SP1). All mentioned messages were bounced with NDR:

5.7.1 Submission has been disabled for this account

It seems to be known issue reported for example by Nuno Mota here. Thx for that!

Basically, it is caused by value 0 KB applied on mailbox quotas despite of using the database quota defaults.

Microsoft Outlook lets you reconfigure a cached mode profile from one use to another user. However, there are known problems if you do this. Therefore, this configuration is not supported.

We recommend and support only one user per Outlook profile in Exchange cached mode. The issues that were reported after you change users in a single profile include the following:

Email may be unexpectedly transferred from one mailbox to another mailbox.

Email may unexpectedly be deleted from one of the mailboxes.

If you have to manage multiple mailboxes administratively in cached mode, you must use a separate profile for each mailbox that you are managing.

Note: Microsoft is not aware of issues that may occur if you use a single online mode profile for multiple users. Although this configuration is supported, if you are using Outlook in online mode, we recommend using a separate profile for each user.

This problem was corrected in Outlook 2013. In Outlook 2013 you cannot change an email account from one user to another user.